INFLUENCE OF RESIDUAL GASES ON CATHODE SPOT BEHAVIOR

A residual gas in a vacuum arc chamber influences the behavior of the arc by two effects: it changes the state of the cathode surface, in particular the surface cleanness, and it influences the interelectrode plasma. Surface contaminations cause a number of additional spot ignition mechanisms and plasma production processes. At higher pressures, the gas hinders the expansion of the cathode spot plasma and interacts with the energetic ions of the spot plasma. Experiments are summarized dealing with the influence of the residual gas on the arc parameters in a pressure range of 10(-6)-10(5) Pa. With increasing pressure, general tendencies are a decrease in the fluctuations of the burning voltage, in the chopping current, in the current density, and an increase in the arc lifetime, spot velocity, and spot diameter. The conditions at the cathode surface are decisive for the spot behavior and not the pressure. Surface contaminations render the arc more stable. The transition between the so-called cathode spot type 1 (on contaminated surfaces) and type 2 (on clean surfaces) was found to be smooth rather than abrupt.